EL34 suppressor grid (g3) characteristics

[Alejandro Lieber]

I am thinking in building an AM transmitter for 7.1 Mhz (40 meters) using a
EL34 in class C as final.
The idea is to modulate it by varying the negative polarity of the
suppressor grid (g3).
Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma with
0 volts suppressor, does anyone have any idea what negative voltage in g3 is
needed to reduce the anode current by half, needed for screen AM modulation ?.

Alejandro Lieber LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map:
http://1fcr.com.ar

--- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---

[Patrick Turner]

On Wednesday, 10 October 2012 10:39:54 UTC+11, Alejandro Lieber wrote:
> I am thinking in building an AM transmitter for 7.1 Mhz (40 meters) using a EL34 in class C as final. The idea is to modulate it by varying the negative polarity of the suppressor grid (g3). Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma with 0 volts suppressor, does anyone have any idea what negative voltage in g3 is needed to reduce the anode current by half, needed for screen AM modulation ?. Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---

Methinks an 807 would be a lot better because 807 has an anode top-cap connection rather than to pin 3 of an octal socket, from where arcs to pin 2 which is the filament usually at earthy potential.

AFAIK, suppressor grid 3 is a bit non linear for modulation on its own. But in you post you wonder what's needed for "screen AM modulation" which I think is much more linear. You should br able to find the set of curves for Ia versus Eg2amoung characteristic curves for EL34 or 807. Usually you will need quite a high voltage swing of maybe 150Vrms for considerable modulation %. And the screen draws DC current so its input impedance is not as high as for a negatively biased grid for AF. Presumably some RF current will be absorbed by the screen. There is plenty info in old UK and USA amateur handbooks on screen modulation. Maybe you will find you will need a another EL34 acting as an SET for audio amp to drive the RF amp screen, i'd guess you'd need a 20H choke feed to anode of audio EL34 and this anode could be directly connected to RF screen with Eg2 supply to choke at say 350V. I'd guess you would need a bypass cap from RF tube screen to 0V so that it operates as a pentode at RF but at AF the bypass cap has no effect. Maybe 200pF might be OK, its reactance at 7MHz = 113 ohms, but at 5kHz its 159k ohms.

Patrick Turner.

[Alex Pogossov]

"Alejandro Lieber" <ali...@notvalid.net> wrote in message
news:k52ck9$1765$1...@adenine.netfront.net...

>I am thinking in building an AM transmitter for 7.1 Mhz (40 meters) using a
>EL34 in class C as final.
> The idea is to modulate it by varying the negative polarity of the
> suppressor grid (g3).
> Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma
> with 0 volts suppressor, does anyone have any idea what negative voltage
> in g3 is needed to reduce the anode current by half, needed for screen AM
> modulation ?.
>
> Alejandro Lieber LU1FCR
> Rosario Argentina

Does not seem like a good choice. EL34 is not for RF. It has high Cag
capacitance = 1pF. You are looking at interdependance of grid and antenna LC
tanks or parasitic self-excitation at worst. And 75W DC input is too much --
unreliable.

Suppressor modulation is not a good choice -- requires about -150V dc and
the same AC component. Nonlinear. Negative g3 voltage repulses the electron
flow to the screen grid. It is easy to burn, unless a series resistor is
used to limit Pg2. (Ask Patrick what happens if you run a push-pull audio
amp without negative feedback at full power and then disconnect its load!)


If you (for whatever strange reason) decide to build AM transmitter (in the
21-st century when even SSB starts to look old fashioned), then use 807 tube
or any other specially designed for RF.

The best modulation is the cathode modulation. For example with 807:
1) get about 35Vm of excitation, connect g1 to ground via a 100K resistor,
apply AC excitation through a cap to g1.
2) Use Vg2= 250...300V
3) Between cathode and GND connect a darlington of a MOSFET with a 100R
resistor from emitter (source) to GND. Bias base (gate) accordingly to get
about 40...50mA DC on idle. Apply AF to base (gate) via a 1uF cap or so. You
will need about 5VmAC. Such transistor, as you can see, works as a
controlled current source. Thus cathode current of your 807 will be
perfectly following the AF input. Cathode of the 807 shall be decoupled to
GND via say 4700pF. Since it is the current that is controlled, this scheme
does not depend on the non-linearity of the curves of the tube. Cathode
potential sets by itself.

Regards,
Alex

[John L Stewart]

'Alejandro Lieber[_3_ Wrote:
> ;963267']I am thinking in building an AM transmitter for 7.1 Mhz (40

As the G3 voltage is biased negative the G2 current will increase in
proportion to the decrease in anode current. That can result in a
dangerous increase in screen dissapation. To prevent that, the screen
needs to be supplied thru a limiting resistance. Refer to the attachment
lifted from FE Terman.

The EL34 will easily handle your 500 volt supply, it is rated to 800
volts on the plate. But the socket is something needs to be of good
insulation. As Patrick T has said, an 807 would be a better choice.

Many amatuer transmitters were built using TV horizontal output tubes
which are still common. That gets around the socket insulation problem.
And it better isolates the RF input circuits from the output. Less
likely to have parasitic oscillations.

Cheers, John


+-------------------------------------------------------------------+
|Filename: Suppressor Modulation B2.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=305|
+-------------------------------------------------------------------+



--
John L Stewart

[Alejandro Lieber]

Patrick Turner wrote:
> On Wednesday, 10 October 2012 10:39:54 UTC+11, Alejandro Lieber wrote:
>> I am thinking in building an AM transmitter for 7.1 Mhz (40 meters)
>> using a EL34 in class C as final. The idea is to modulate it by varying
>> the negative polarity of the suppressor grid (g3). Suppose I load the
>> EL34 plate with 500 volts DC and a current of 150 ma with 0 volts
>> suppressor, does anyone have any idea what negative voltage in g3 is
>> needed to reduce the anode current by half, needed for screen AM
>> modulation ?. Alejandro Lieber LU1FCR Rosario Argentina Real-Time
>> F2-Layer Critical Frequency Map: http://1fcr.com.ar ---
>> news://freenews.netfront.net/ - complaints: ne...@netfront.net ---
>
> Methinks an 807 would be a lot better because 807 has an anode top-cap
> connection rather than to pin 3 of an octal socket, from where arcs to
> pin 2 which is the filament usually at earthy potential.

The idea of using suppressor modulation is that no audio power is needed.
The 807 has no suppressor, and screen grid (g2) AM modulation is very
nonlinear in the 807. It works much better with a 6146 or 6DQ5 or 6DQ6.

> AFAIK, suppressor grid 3 is a bit non linear for modulation on its own.
> But in you post you wonder what's needed for "screen AM modulation" which

I should have written: "suppressor AM modulation"

> I think is much more linear. You should br able to find the set of curves
> for Ia versus Eg2amoung characteristic curves for EL34 or 807. Usually
> you will need quite a high voltage swing of maybe 150Vrms for
> considerable modulation %. And the screen draws DC current so its input
> impedance is not as high as for a negatively biased grid for AF.
> Presumably some RF current will be absorbed by the screen. There is
> plenty info in old UK and USA amateur handbooks on screen modulation.
> Maybe you will find you will need a another EL34 acting as an SET for
> audio amp to drive the RF amp screen, i'd guess you'd need a 20H choke
> feed to anode of audio EL34 and this anode could be directly connected to
> RF screen with Eg2 supply to choke at say 350V. I'd guess you would need
> a bypass cap from RF tube screen to 0V so that it operates as a pentode
> at RF but at AF the bypass cap has no effect. Maybe 200pF might be OK,
> its reactance at 7MHz = 113 ohms, but at 5kHz its 159k ohms.
>
> Patrick Turner.

As you know, ham radio is about experimenting. Although many hams still use
valves in AM transmitters, no one uses suppressor modulation.
45 years ago, I did my military service, and was in charge of repairing all
the old valve transmitters. A lot used the 2E22 a 30 watt anode valve
specially built for AM suppressor modulation. I think a 3A4 was used as
final in the audio amplifier.
But now I discover that the 2E22 like the 803. both built for suppressor
modulation, are capable of very high screen dissipation, as very high screen
current appear when the suppressor is very negative polarizes.

--

[Alejandro Lieber]

John L Stewart wrote:
> 'Alejandro Lieber[_3_ Wrote:
>> ;963267']I am thinking in building an AM transmitter for 7.1 Mhz (40
>> meters) using a
>> EL34 in class C as final.
>> The idea is to modulate it by varying the negative polarity of the
>> suppressor grid (g3).
>> Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma
>> with
>> 0 volts suppressor, does anyone have any idea what negative voltage in
>> g3 is
>> needed to reduce the anode current by half, needed for screen AM
>> modulation ?.
>>
>> Alejandro Lieber LU1FCR
>> Rosario Argentina
>>
>> Real-Time F2-Layer Critical Frequency Map:
>> http://1fcr.com.ar
>>
>> --- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---
>
> As the G3 voltage is biased negative the G2 current will increase in
> proportion to the decrease in anode current. That can result in a
> dangerous increase in screen dissapation. To prevent that, the screen
> needs to be supplied thru a limiting resistance. Refer to the attachment
> lifted from FE Terman.

I have discovered that the 2E22 like the 803. both built for suppressor

modulation, are capable of very high screen dissipation, as very high screen
current appear when the suppressor is very negative polarizes.

--

[Alejandro Lieber]

Alex Pogossov wrote:
> "Alejandro Lieber" <ali...@notvalid.net> wrote in message
> news:k52ck9$1765$1...@adenine.netfront.net...
>> I am thinking in building an AM transmitter for 7.1 Mhz (40 meters) using a
>> EL34 in class C as final.
>> The idea is to modulate it by varying the negative polarity of the
>> suppressor grid (g3).
>> Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma
>> with 0 volts suppressor, does anyone have any idea what negative voltage
>> in g3 is needed to reduce the anode current by half, needed for screen AM
>> modulation ?.
>>
>> Alejandro Lieber LU1FCR
>> Rosario Argentina
>
> Does not seem like a good choice. EL34 is not for RF. It has high Cag
> capacitance = 1pF. You are looking at interdependance of grid and antenna LC
> tanks or parasitic self-excitation at worst. And 75W DC input is too much --
> unreliable.

I have seen the EL34 used as exciter for 200 / 800 watts transmitters.

A 75W DC input in class C with 66% efficiency will dissipate 25W in the EL34
anode, still below it's capabilities.

> Suppressor modulation is not a good choice -- requires about -150V dc and
> the same AC component. Nonlinear. Negative g3 voltage repulses the electron
> flow to the screen grid. It is easy to burn, unless a series resistor is
> used to limit Pg2. (Ask Patrick what happens if you run a push-pull audio
> amp without negative feedback at full power and then disconnect its load!)
>
>
> If you (for whatever strange reason) decide to build AM transmitter (in the
> 21-st century when even SSB starts to look old fashioned), then use 807 tube
> or any other specially designed for RF.
>
> The best modulation is the cathode modulation. For example with 807:
> 1) get about 35Vm of excitation, connect g1 to ground via a 100K resistor,
> apply AC excitation through a cap to g1.
> 2) Use Vg2= 250...300V
> 3) Between cathode and GND connect a darlington of a MOSFET with a 100R
> resistor from emitter (source) to GND. Bias base (gate) accordingly to get
> about 40...50mA DC on idle. Apply AF to base (gate) via a 1uF cap or so. You
> will need about 5VmAC. Such transistor, as you can see, works as a
> controlled current source. Thus cathode current of your 807 will be
> perfectly following the AF input. Cathode of the 807 shall be decoupled to
> GND via say 4700pF. Since it is the current that is controlled, this scheme
> does not depend on the non-linearity of the curves of the tube. Cathode
> potential sets by itself.

Very interesting. Do you know if it has been done in practice ?
Will the final valve still work in class C all the modulation cycle ?

I also remember seeing a rig using 2 6W6 (or 6Y6) in parallel in the cathode
of the RF output valve.


--

Alejandro Lieber LU1FCR
Rosario Argentina

[Patrick Turner]

On Wednesday, 10 October 2012 18:48:54 UTC+11, John L Stewart wrote:
> 'Alejandro Lieber[_3_ Wrote: > ;963267']I am thinking in building an AM transmitter for 7.1 Mhz (40 > meters) using a > EL34 in class C as final. > The idea is to modulate it by varying the negative polarity of the > suppressor grid (g3). > Suppose I load the EL34 plate with 500 volts DC and a current of 150 ma > with > 0 volts suppressor, does anyone have any idea what negative voltage in > g3 is > needed to reduce the anode current by half, needed for screen AM > modulation ?. > > Alejandro Lieber LU1FCR > Rosario Argentina > > Real-Time F2-Layer Critical Frequency Map: > http://1fcr.com.ar > > --- news://freenews.netfront.net/ - complaints: ne...@netfront.net --- As the G3 voltage is biased negative the G2 current will increase in proportion to the decrease in anode current. That can result in a dangerous increase in screen dissapation. To prevent that, the screen needs to be supplied thru a limiting resistance. Refer to the attachment lifted from FE Terman. The EL34 will easily handle your 500 volt supply, it is rated to 800 volts on the plate. But the socket is something needs to be of good insulation. As Patrick T has said, an 807 would be a better choice. Many amatuer transmitters were built using TV horizontal output tubes which are still common. That gets around the socket insulation problem. And it better isolates the RF input circuits from the output. Less likely to have parasitic oscillations. Cheers, John +-------------------------------------------------------------------+ |Filename: Suppressor Modulation B2.jpg | |Download: http://www.audiobanter.com/attachment.php?attachmentid=305| +-------------------------------------------------------------------+ -- John L Stewart

Amateurs I have known used to use 807 mainly for all their RF amps and AF modulators because they were cheap from Army disposal stores and had better insulation for anode connections.
But one ham said that a pair of EL36/6CM5 ( TV line output )could make 300Watts in class C. Other octal line outputs with anode top caps were 6DQ6 and 6CD6.
Then there was the 6146 which many hams used.

Very few SE transmitters were made but I remember buying a very much modded ex Army tranciever known as the "22 set" which had just one 807 as its RF output tube, and might have have used the audio 6V6 output for a modulator. But its now 50 years (1962) since I had that set which perished because the roof leaked very badly in the shack where I'd had it. Roof was made of "Malthoid", a horrible tar impregnated cardboard meant for sarking under roof tiles, and not meant for the roof itself, but in 1946, people used all sorts of stuff because better roofing wasn't available, and everyone was so penny pinching, including my dad, even though he was a vet. They wouldn't let him fight, so he was well off during WW2, but the shack was temporary because he assisted a number of Euro migrants for 10 years after the war - long story, and shack with mud walls is still there, but timbers are a bit wrecked after the 10 years of wet and rot.

I lost all interest in nerdy electronics by around 1964. I repaired the roof in about 1976, using some new "Fibro" sheeting in the form of diamond pattern roof times over the rafters and pine lining boards. Shack was real hot in summer, cold in winter. I cleaned out all the rusty old rubbish. I'm a bit rusty on RF stuff now, but I did at one stage have a separate receiver to listen to a friend on one F and using headphones, while using another F to transmit on, so it was like a telephone. I recall wrecking at least one old vintage radios to make a transmitter using one 6V6 and its old radio OPT to modulate the cathode of a 6V6 used as an RF amp which used a spare RF SW coil and one tuning gang, which oscilator signal taken from the existing mixer oscilator. In 1962 I guess my mate and I were ready for WW3 and could set up emergency communications if we had access to 240Vac, which of course wouldn't have been available if Russia had let off a 1 Megatonne bomb a kilometre above Sydney. WW3 was in all our minds because it wasn't long after the Cuban missile crisis. Then JFK got shot, and we thought anything was possible........

But i don't recall anyone using suppressor grid modulation. Real men used plate modulation because then the modulator power adds to the power made by the RF amp. My friend bought a "gigantic" ex army set which sat on a pair of empty 44 gallon drums, with 6 pull-out rack units and 807 modulation tubes plus 2 x 813 RF outputs working in 2,500Vdc. But he too secumbed to the distractions of the world, and the shed his father built also rotted to peices while he spent years living as a hippy up north. That set collapsed through the rotting floor of the shed, laid on its side in mud for 15 years, then stashed under the main house. His parents eventually died, and I heard he's got the set working again, after settling down at age 58, without completely secumbing to the ganga. I now prefer bicycles to motorcycles, and oh how life has changed in 50 years, but I do have a collection of radios and transmitters I was given after one ham I knew died about 7 years ago. If ever I move up from AF to RF, I have a pile of nice old communications stuff to repair/restore/get working etc.
But In 1994, I did make myself an RF gene with a 6BX6 used as RF amp and with control grid 1 modulation. It uses a fairly large value Rk bypassed for RF, but not for AF, thus Ia change is linearized and the modulation was fairly linear up to 80%. I later added an AF detector which then fed the AF detected back to a differential amp with 2 triodes used for audio amp so that there is NFB, and this made 95% modulation possible with maybe only 1.5% THD to the audio envelope, for all AF between say 20Hz and 3kHz, enough to test many old radios.
I even added a saw tooth generator to vary voltage across 10 x 68V 5 watt zener diodes used as varicaps for the oscilator. Used properly, the zeners make good varicap diodes capable of the high voltages found in grid circuits of tubed oscillators. I can get +/- 40kHz at 455kHz centre F.

Patrick Turner.

[Alejandro Lieber]

My first ham transmitter (1969) used only two valves: a EL36 from an old TV
set and a 6V6. The EL36 was a crystal controlled (3.640 Mhz) oscillator with
10 watts output, plate modulated by the 6V6 with a reactor. The microphone
was an old carbon telephone one with the DC current taken from the cathode
of the 6V6, and a 3.2 ohm to 10.000 ohm small transformer as voltage
amplifier. No power transformer, 300 V DC directly from the 220 V AC line.
I made hundreds of contacts up to 700 km. Much more fun than what I get now
with my commercial Single Side Band rig.

>........
> Patrick Turner.


--

[Patrick Turner]

On Friday, 12 October 2012 01:44:25 UTC+11, Alejandro Lieber wrote:
> Patrick Turner wrote: > On Wednesday, 10 October 2012 18:48:54 UTC+11, John L Stewart wrote: >> 'Alejandro Lieber[_3_ Wrote: > ;963267']I am thinking in building an AM >> transmitter for 7.1 Mhz (40 > meters) using a > EL34 in class C as >> final. > The idea is to modulate it by varying the negative polarity of >> the > suppressor grid (g3). > Suppose I load the EL34 plate with 500 >> volts DC and a current of 150 ma > with > 0 volts suppressor, does >> anyone have any idea what negative voltage in > g3 is > needed to >> reduce the anode current by half, needed for screen AM > modulation ?. >> > > Alejandro Lieber LU1FCR > Rosario Argentina > > Real-Time F2-Layer >> Critical Frequency Map: > http://1fcr.com.ar > > --- >> news://freenews.netfront.net/ - complaints: ne...@netfront.net --- As >> the G3 voltage is biased negative the G2 current will increase in >> proportion to the decrease in anode current. That can result in a >> dangerous increase in screen dissapation. To prevent that, the screen >> needs to be supplied thru a limiting resistance. Refer to the >> attachment lifted from FE Terman. The EL34 will easily handle your 500 >> volt supply, it is rated to 800 volts on the plate. But the socket is >> something needs to be of good insulation. As Patrick T has said, an 807 >> would be a better choice. Many amatuer transmitters were built using TV >> horizontal output tubes which are still common. That gets around the >> socket insulation problem. And it better isolates the RF input circuits >> from the output. Less likely to have parasitic oscillations. Cheers, >> John >> +-------------------------------------------------------------------+ >> |Filename: Suppressor Modulation B2.jpg | |Download: >> http://www.audiobanter.com/attachment.php?attachmentid=305| >> +-------------------------------------------------------------------+ >> -- John L Stewart > > Amateurs I have known used to use 807 mainly for all their RF amps and AF > modulators because they were cheap from Army disposal stores and had > better insulation for anode connections. But one ham said that a pair of > EL36/6CM5 ( TV line output )could make 300Watts in class C. Other octal > line outputs with anode top caps were 6DQ6 and 6CD6. Then there was the > 6146 which many hams used. My first ham transmitter (1969) used only two valves: a EL36 from an old TV set and a 6V6. The EL36 was a crystal controlled (3.640 Mhz) oscillator with 10 watts output, plate modulated by the 6V6 with a reactor. The microphone was an old carbon telephone one with the DC current taken from the cathode of the 6V6, and a 3.2 ohm to 10.000 ohm small transformer as voltage amplifier. No power transformer, 300 V DC directly from the 220 V AC line. I made hundreds of contacts up to 700 km. Much more fun than what I get now with my commercial Single Side Band rig. >........ > Patrick Turner. -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---

I remember lots of ppl were into ham radio. But during these last years, whenever I have tuned in to someone, he was usually older than me, maybe 80, and spending the whole time talking about his latest hospital operation, droning on and on and on, and ppl were expected to wait until he'd finished, then someone else would give a big long lecture about his medical conditions. It seemed to me ham radio wasn't about really connecting to other men for a useful reason, but was about being a technical nerd and narcissist. I can tell you this mode of human existance suits many blokes who are very ill at ease when having any real deep and meaningful personal interactions with anyone else. Maybe WW2 fucked them up pretty bad. Besides, ham radio offers a chance to escape the missus and kids while you spent 20 years in suburbia raising a family after 1 month's bliss with a young missus, then 30 years waiting to die. The ham radio shed was no place for the missus or kids, or maybe even grand kids, so often very full of cigarette smoke from the chain smoking ham, who ignored his health conditions, and expected doctors to do miracles. I think I found out early the basic trouble with ham radio is that its like listening to Castro giving a 3 hour lecture on wonderful socialism on Cuban radio, and sure, nothing wrong with socialism yet soon after it is implemented all human progress is stunted and most humans die of boredom and frustration. Mind you, listening to Romney is alarming, worse than Castro. Just as well these silly old dinosaurs didn't get into ham radio, they'd be able to drone on for hours about how they fucked everything up, or how they were about to fuck everything up, while never ever admitting how they were fucking things up right now.

But yeah, 6CM5 plus a 6V6 and telephone mic probably could sound good and go far. Just like you need a horn speaker with a 9 watt AF amp, you need a good antenna for 9 watts of RF. I helped a few hams erect huge rotating beam antennas above their houses. Some had elements that overhung the house. Neighbours were always appalled seeing these things going up on the house next door, and they complained about TVI and RFI even when the ham wasn't transmitting.

As I mentioned in a previous post, my friend in 1962 had an 800Watt transmitter with 2 x 813 outputs. He operated as a ham for many years, a rare young man although very much a young nerd - but who had handsome features so he was a hit with the girls, and once he quite casually stole a GF from me. But having a motorcycle before I could get one was a huge boost to getting pussy. Of course I soon realised she was just another air-head deadbeat tart, and very hard to get along with like most other shielas at the time who were so often wild and bored and unable to settle down, or able to cook or even do a good BJ. Anyway, my friend had this weatherboard shed with flat corrugated iron roof. The iron and boards and door and one window were then probably 50 years old and all been taken from a small cottage that had been demolished nearby maybe in 1958. But this iron had spent its life under gum trees and settling leaves had taken their toll on the zinc. My friend's old man was a bank manager and extremely tight with money, so getting new iron was out of the question. So once the old iron was very worn and disturbed and re-used in yet another damn flat roof under overhanging trees it quickly gathered leaves and leaked a lot, and there wasn't any silicone in them days and the friend constantly battled leaks and dampness in the shed. Of course the friend wasn't too law abiding, and operated as a pirate ham by addopting the call sign of someone local who'd died about 5 years before, sufficiently long ago for most ppl to have completely forgotten he existed. But one day my friend said somebody was returning his CQ, and said "Ah John, good to hear you are back on air again, someone said you had died..." and my friend had to be diplomatic and make up a story then and there, and then say dinner was ready and the missus was calling out, so "over and out..."
My friend didn't want to learn morse code, an essential requirement to get a ham radio license. This regulation stopped hundreds of people who would have liked to become a ham radio enthusiast. Governments acted as Giant Kill Joy departments in 1960.
My friend's ex-army radio set transmissions attracted the attention of the PMG, Post Master General or post office department which also regulated and controlled all radio and TV transmissions. My friend worked as a telephone linesman in the PMG after leaving school earlier than me, and he knew about how other sections worked, and about their radio detection van which drove around to find illegal radio transmissions. Well, one day while transmitter was on, he watched the van drive right past his house. He was using the shed roof iron as antenna, about 3 metres x 4 metres, flat. Electronically, it was very poor, but with such high power the RF still went far. I recall the antenna lead from Xmitter just went out the door with large aligator clip to roofing iron, and a light globe was used to see if it was tuned right. Later he spoke to blokes in the van, and they said they'd "detected a very powerful signal in Warrangi St, but we couldn't get a proper fix on it and we couldn't see any antenna.." My friend knew his set overloaded their gear. He could hardly keep a straight face.
Patrick Turner.

[Alex Pogossov]

"Alejandro Lieber" <ali...@notvalid.net> wrote in message

news:k54s5k$2r6g$1...@adenine.netfront.net...

> Very interesting. Do you know if it has been done in practice ?
> Will the final valve still work in class C all the modulation cycle ?
>
> I also remember seeing a rig using 2 6W6 (or 6Y6) in parallel in the
> cathode of the RF output valve.

Cathode modulation has more advantages.

1. It is a sum of control grid modulation and partly screen modulation. As
the cathode voltage rises at the troughs of modulation, voltage difference
between the screen and cathode reduces. Thus the "pull" of the screen grid
reduces which cooperates with the effect of the cutting of by the control
grid. Thus the cathode voltage excursions are smaller than control grid
excursions if it were purely grid modulation.

2. Modulation is not affected by variation of excitation voltage. In a
common grid modulation, if the excitation voltage say is modulated by 50Hz
hum, output will also be modulated. With the proposed cathode current
modulation output will not be affected. Cathode voltage might be modulated
by hum, tracking it like in a cathode detector, but since the current is
stable, the anode RF component will be almost unchanged (ignoring cut-off
angle effects).

But the greatest advantage is no need for any adjustments of bias, etc. The
only thing to be set is the cathode current. It is done by adjusting bias on
the gate (base) of the modulator series transistor.

Regards,
Alex

[Alejandro Lieber]

Very interesting.
Where can I look for a practical circuit ?

[Alejandro Lieber]

Patric:
I am an electronic engineer. Studied here in my town, Rosario, Argentina.
Being a ham helped me a lot while studying. But in the last 20 years, I have
talked to the microphone no more than 20 hours. I mostly tune to 7 Mhz, the
last band where some hams can be found, most from no more than 500 km,
mostly from places I have visited during my life. I like to hear hams
talking while I work on the computer. I rarely talk to them, mostly because
the useful time one looses.

[Alex Pogossov]

"Alejandro Lieber" <ali...@notvalid.net> wrote in message

news:k5aemj$11pc$1...@adenine.netfront.net...

Nowhere. The whole fun is to create this circuit and experiment. Patrick has
done similar thing (cathode current modulation) in his low distortion test
AM modulator. You have a chance to try in a higher wattage range. I did it
many years ago with a smaller tube, similar to 6BW6, for a 10m band
transmitter.

Regards,
Alex

[patrick-turner]

On Saturday, 13 October 2012 16:12:47 UTC+11, Alex Pogossov wrote:
> "Alejandro Lieber" <ali...@notvalid.net> wrote in message news:k5aemj$11pc$1...@adenine.netfront.net... > Alex Pogossov wrote: >> "Alejandro Lieber" <ali...@notvalid.net> wrote in message >> news:k54s5k$2r6g$1...@adenine.netfront.net... >>> Very interesting. Do you know if it has been done in practice ? >>> Will the final valve still work in class C all the modulation cycle ? >>> >>> I also remember seeing a rig using 2 6W6 (or 6Y6) in parallel in the >>> cathode of the RF output valve. >> >> Cathode modulation has more advantages. >> >> 1. It is a sum of control grid modulation and partly screen modulation. >> As >> the cathode voltage rises at the troughs of modulation, voltage >> difference >> between the screen and cathode reduces. Thus the "pull" of the screen >> grid >> reduces which cooperates with the effect of the cutting of by the control >> grid. Thus the cathode voltage excursions are smaller than control grid >> excursions if it were purely grid modulation. >> >> 2. Modulation is not affected by variation of excitation voltage. In a >> common grid modulation, if the excitation voltage say is modulated by >> 50Hz >> hum, output will also be modulated. With the proposed cathode current >> modulation output will not be affected. Cathode voltage might be >> modulated >> by hum, tracking it like in a cathode detector, but since the current is >> stable, the anode RF component will be almost unchanged (ignoring cut-off >> angle effects). >> >> But the greatest advantage is no need for any adjustments of bias, etc. >> The >> only thing to be set is the cathode current. It is done by adjusting bias >> on >> the gate (base) of the modulator series transistor. >> > Very interesting. > Where can I look for a practical circuit ? Nowhere. The whole fun is to create this circuit and experiment. Patrick has done similar thing (cathode current modulation) in his low distortion test AM modulator. You have a chance to try in a higher wattage range. I did it many years ago with a smaller tube, similar to 6BW6, for a 10m band transmitter. Regards, Alex > > -- > Alejandro Lieber LU1FCR > Rosario Argentina > > Real-Time F2-Layer Critical Frequency Map: > http://1fcr.com.ar > > --- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---

One small tube which might be good for G3 AM modulation could be the small mini 7 pin 6DT6, because it was common for an FM demodulator using quadrature detection, but fairly simple, and 6DT6 was made to have good G3 gm character.
Once the RF is AM modulated, it can just be amplified by a following RF output tube, maybe 1 x 807 is good.

Patrick Turner.

[flipper]

The 6DT6 is a "dual control pentode," g3 being the 'second' control,
and is the current 'tube of choice' for what has become the defacto
'standard' BCB AM transmitter on www.antiqueradios.com (Homebrew
Radios), because it is a 'dollar days' tube. That, and other
variations, trace back to Norm's 6888 transmitter. The 6888 and it's
loctal equivalent the 7ak7 are the most 'powerful' of the lot. The
7ak7 is also a 'dollar days' tube but being loctal seems to nix it's
popularity for some reason.

Prior to the 6DT6 the 6GY6 was the 'tube of choice' because it's
cheaper but allegedly produces almost the same output level as the
6888 did.

The 6GX6, 6HZ6, and 6AS6 are also dual control pentodes and the
compactron 6BV11 has two dual control pentodes in the one bottle.

The 'standard' configuration is a solid state brick oscillator
exciting G1 with audio, after a triode preamp, to G3. Norm's original
used an input transformer and the higher cathode bias needed was
enough to power the brick under the 6888.

Since there are only 2 readily available brick oscillators in the BCB
there are also tunable versions.

The current 'tunable' 6DT6 schematic is here

http://amradio.freeiz.com/transmitter/6bk76dt6.jpg

I think this is the 'original' 6888 thread (although in a previous
thread Norm is 'working on it')
http://antiqueradios.com/forums/viewtopic.php?t=63310

And the follow up when it was built
http://antiqueradios.com/forums/viewtopic.php?t=63526

Of course, there are dual control (when sharp cutoff) heptodes too.
Here's one using a 6AW8 as local osc, feeding the 6CS6, and RF final.
http://home.comcast.net/~suptjud/6CS66LQ8Transmitter.JPG

This is the 'standard' antiqueradios version, which is the same thing
as the dual control pentodes but using the 12CS6.
http://amradio.freeiz.com/12av612cs6rev2.jpg

And then there are 'twin pentodes', like the 6LE8, 6HS8, 6GS8,6BU8,
with common g1 and k that, when g3 and plate are tied together are
dual control pentodes.

Lots of g3 modulation.

Probably the most ingenious is this one
http://www.radiomuseum.org/forum/hi_fi_am_transmitter.html
He subtracts screen from plate to keep modulation wholly on the
'linear' portion of the G3 curve.

Another interesting transmitter is Bob Weaver's screen modulation with
carrier control and while that's not g3 modulation I mention it
because it's sort of a counterpoise to the above 'hi-fi' idea since
compression *is* distortion. But why go to all that linearization if
you're going to compress in the preamp?
http://www.antiqueradios.com/forums/viewtopic.php?f=12&t=170605
He also built one using a 6V6.

[Alejandro Lieber]

I remember seeing some 40 years ago a 1 watt one tube AM short wave
transmitter using suppressor modulation using a single 6BH6.